Time Systems and Dates - GPS Time

GPS TIME

Occasionally there is need to use the details of how GPS
labels time tags. In general this does not come up for the
casual user. The GPS message contains information that allows a
receiver to convert GPS Time into Universal Time (really UTC or
Universal Time Coordinated) or any time zone. Universal Time
used to be called Greenwich Mean Time or Zulu time.

GPS Time is a uniformly counting time scale beginning at the
1/5/1980 to 1/6/1980 midnight. January 6, 1980 is a Sunday.
GPS Time counts in weeks and seconds of a week from this
instant. The weeks begin at the Saturday/Sunday transition.
The days of the week are numbered, with Sunday being 0, 1
Monday, etc.

GPS week 0 began at the beginning of the GPS Time Scale.
Within each week the time is usually denoted as the second of
the week. This is a number between 0 and 604,800 ( 60 x 60 x 24x 7).
Sometimes the second-of-week (SOW) is split into a day of
week (DOW) between 0 and 6 and a second of day (SOD) between 0
and 86400.

The word "uniformly" is used above to indicate that there are
no "leap seconds" in this time system. The world does not
rotate smoothly. This can easily be measured with atomic clocks.
A plot of the difference between atomic clock time and "earth"
time shows a slow drift with some small irregularities.
Below is a plot of the difference between earth rotation time (UT1) and
the atomic time used by science - UTC. The jumps are where UTC was
adjusted to keep it within a second of earth rotation time. (Leap Seconds
were introduced there.)

The current slow drift is not constant over decades and comes back.
The source of these variations are many, some unknown. Tidal
effects can be predicted over centuries, but there have been
changes due to things like the 1990's el Nino event and even
large geomagnetic storms.

A data set available at the
IERS
goes back to 1623. It is based on an
analysis of astronomical events and historic data. The data after 1860
is thought to be reliable and after 1956 very good atomic clock data was
used. Here are the difference in the length of a day, in milliseconds,
and the accumulated difference from the value in 1800.

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This is based on the work of Morrison of the Royal Greenwich Observatory.

Every year or so, the "world as a clock" is about a
second off from atomic time. Since 1972 the world timing
community has agreed to insert or take out 1 second of time in
the UTC time scale to keep the earth rotation aligned to within
a second of atomic time. This is done on either January 1 or
July 1 of a given year. To date there have only been extra
seconds inserted, none taken out. This is like the insertion of
February 29 into leap years. Therefore these have been called
leap seconds. A detailed discussion of leap seconds can be
found on the
a US Naval Observatory WWW site.
The jumps in the plot are due to leap seconds changing UTC.

Because GPS time does not have leap seconds, it will change
by 1 second with respect to UTC whenever a leap second is
inserted. The two time scales were aligned when GPS time began
on January 6, 1980. With the next insertion of a leap second,
on July 1, 1981, the UTC time scale began reporting a number 1
second smaller than GPS time. Up to 2000, there have been 13
leap seconds.

GPS to UTC Offset

Beginning

GPS - UTC

On Date

(sec)

Jan 6 1980

0

Jul 1 1981

1

Jul 1 1982

2

Jul 1 1983

3

Jul 1 1985

4

Jan 1 1988

5

Jan 1 1990

6

Jan 1 1991

7

Jul 1 1992

8

Jul 1 1993

9

Jul 1 1994

10

Jan 1 1996

11

Jul 1 1997

12

Jan 1 1999

13

The navigation message of the GPS satellites contains the
number of seconds offset between GPS and UTC time. It also has
a note on when the next leap second will be inserted, if known.
In general the insertion of a leap second is only decided about
2 months before it is done. After all, it is based on the
anomalies in the earth's spin rate, which are not predictable,
but only measurable.

UT/UTC

The abbreviation UT is not used consistently in popular
literature. To an astronomer it means a special time frame,
called UT1, that is determined by the real spinning of the
earth. In a lot of non-scientific literature, it means UTC
which used to be called Greenwich Mean Time (GMT).

UTC stands for Universal Time Coordinated. It is what you
hear from time stations WWV, CHU etc. It is a time frame based
on atomic clocks and aligned with the zero longitude time zone.
UTC, plus an offset for time zones, is what we use in everyday
life. UT1 is principally of interest to astronomers who have to
know the precise orientation of the earth for observations.

(It's use in GPS is important, but hidden from the user. The satellites
are really positioned in inertial space and the earth orientation is needed
to use them to position a place on the earth. UT1, and polar motion, are
needed for this at the 500 m level. All the work to do this is done at
the operation control center and included in the Broadcast Ephemeris that
are uploaded. These generate satellite positions in earth fixed coordinates
using GPS Time as an input.)

Julian Date - Modified Julian Date

In everyday life we use month, day and year to denote a date.
However adding and subtracting dates is complicated.
Astronomers use a time scale that counts uniformly in days they
call the Julian Date. They set the origin of this date system
far back in time, at January 0, 4173 BC at noon. This scale is
counted in days and fractions of a day. January 1, 2000 at noon
will be JD = 2,451,545.0 . The days used to be started at
noon, hence the use of noon as the reference time. In fact,
astronomical publications used noon as the day dividing line
until 1925.

Julian dates are large numbers and sometime hard to handle.
There is a slightly modified JD scale that is commonly used in
science today. It is called the Modified Julian Date (MJD).
The introduction of this scale was used to place the day
transition at midnight, agreeing with the civilian days. MJD is
defined as the Julian Date minus 2,400,000.5 . Thus January 1,
2000 at noon will be MJD of 51,544.5 .